1. Field of the Invention
The present invention generally relates to a cathode ray tube for use in, for example, a television receiver set, a computer terminal monitor display or the like. More particularly, to the cathode ray tube of a type having an anode button embedded in a funnel section of an envelope of the cathode ray tube for electric connection with an internal electroconductive coating disposed on the internal surface of at least the funnel section of the envelope.
2. Description of the Prior Art
As shown in FIG. 1 of the accompanying drawings, which illustrates a popular color cathode ray tube (a side view with a portion cut away), the well-known color cathode ray tube comprises a highly evacuated envelope 1 including a generally conical funnel section 3 having a large-sized end closed by a faceplate 2 and a small-sized end continued to a generally cylindrical neck section 4. It further contains an electron gun assembly 13 housed within the neck section 4 at one end thereof opposite to the funnel section 3. The faceplate 2 has an inner surface deposited with a luminescent phosphor screen 5 formed of a predetermined pattern of primary color elemental phosphor deposits, for example, triads of red, blue and green phosphor dots.
The cathode ray tube also comprises a color selection electrode or shadow mask 6 which is a perforated thin metal foil having a predetermined pattern of apertures which can be triads of minute circular holes. Which the pattern corresponds to the pattern of the primary color elemental phosphor deposits on the luminescent phosphor screen 5. This shadow mask 6 is supported by a frame structure 7 which is also used to secure the shadow mask immovably inside the faceplate 2 while spaced a predetermined distance from the luminescent phosphor screen 5. On one side of the shadow mask 6, opposite to the luminescent phosphor screen 5, an internal magnetic shield 9 is secured to and supported by the frame structure 7 within the funnel section 3, for shielding the interior of the envelope 1 from an adverse influence which may be brought about by an external magnetic field such as originating from, for example, terrestrial magnetism.
At least the funnel section 3 has its inner surface deposited with an internal electroconductive coating 8 which is formed by applying a paint of graphite to the inner surface thereof. This internal electroconductive coating 8 is electrically connected with the frame structure 7 through an elastic electroconductive member 10, which may be a metal leaf spring. It has one end secured to the frame structure 7 and the other end held in contact with the internal electroconductive coating 8. This elastic electroconductive member 10 serves to feed a high voltage of, for example, 20 to 30 Kv, applied from an external power source to the internal electroconductive coating 8, to the shadow mask 6 through the frame structure 7. The supply of the high voltage, from the external power source to the internal electroconductive coating 8 and then to the shadow mask 6 through the elastic electroconductive member 10 and also through the frame structure 7, is carried out through a generally cup-shaped anode button 11 made of electroconductive material. The details of the button 11 will now be described with particular reference to FIG. 2 illustrating the anode button 11, in a side sectional view representation in an enlarged scale.
According to the prior art, as best shown in FIG. 2, the anode button 11 is tightly inserted in a mounting hole 3a defined in the funnel section 3 of the cathode ray tube and extending completely through the thickness of the wall of the funnel section 3. This anode button 11 is of a generally cup-like configuration, as hereinbefore described, having a bottom wall 11b held in contact with the internal electroconductive coating 8 when, and so long as, the anode button 11 is fitted into the mounting hole 3a. The anode button 11 also has an opening 11a defined therein in opposition to the bottom 11b thereof for receiving therein, a forked contact element 12 (See FIG. 1). The element 12 is utilized for the application of the high voltage from the external power source to the internal electroconductive coating 8 through the anode button 11.
The anode button 11 illustrated in FIG. 2 is prepared into the generally cup-like configuration by the use of a press work from a metallic plate having a thickness of, for example, 0.45 in thickness and made of, for example, Fe-Ni-Cr alloy. Within the anode button 11, a retaining ring member 14, having a connector opening 14a defined therein, is accommodated and integrated with the anode button 11 such that the forked contact element 12 shown in FIG. 1 can be detachably engaged, in a manner substantially shown in FIG. 4, to the retaining ring member 14 through the opening 11a and then through the connector opening 14a.
With the conventional cathode ray tube so constructed as hereinabove described, the application of a predetermined voltage to individual electrodes of the electron gun assembly 13 and also the application of the high anode voltage of, for example, 20 to 30 Kv to the internal electroconductive coating 8 through the anode button 11 in the manner as hereinabove described, result in the eventual reproduction of a picture through the luminescent phosphor screen 5. During the operation of the cathode ray tube in this manner, it is well known that X-rays are generated inside the envelope 1 as electron beams radiated from the electron gun assembly 13 impinge upon the shadow mask 6 and the luminescent phosphor screen 5. This X-radiation emission is known to increase with an increase of the applied anode voltage. Leakage of the X-rays from the cathode ray tube to the surroundings results in a hazardous condition to which living bodies should not be exposed. Accordingly, protective counter-measures have hitherto been taken against the X-radiation emission from the cathode ray tube along with the implosion protection of the cathode ray tube.
FIG. 3 of the accompanying drawing illustrates a characteristic curve (generally known as a X-radiation dose limit curve) showing the relationship between the anode voltage at an anode current of 300 microamperes and the X-radiation dose rate. The characteristic curve corresponds to the maximum the X-ray radiation dose rate of 0.5 mR/H as recommended by ICRP (International Committee for Radiation Protection) from the cathode ray tube at a place 5 cm distant from the front face of the faceplate 2. This X-radiation dose limit curve is formulated by a cathode ray tube manufacturer and registered in EIAJ (Electronic Industry Association of Japan) as a guideline which television receiver manufacturers refer to in designing television receiver sets.
Referring still to FIG. 3, the X-ray radiation dose rate through the faceplate 2 is indicated by a line A. The X-ray radiation dose rate through the evacuated envelope 1 except the anode button 11 is indicated by a line B. The X-ray radiation dose rate through the anode button 11 is indicated by a line C. Comparing these lines A, B and C, it is clear that the X-ray radiation dose rate is greatest through the anode button 11. In other words, greater X-radiation emission is found through the anode button 11 than through any other portions of the evacuated envelope of the cathode ray tube.
In order to minimize the X-radiation emission from the anode button 11, various attempts have been made. One of them is shown in FIG. 4 and includes the use of a metallic shield plate 15, effective to shield X-rays, being welded in overlapping relation to the bottom 11b of the anode button 11. Another one is shown in FIG. 5 and includes the use of an anode cap 16, made of silicone rubber, for exteriorly covering the anode button 11, together with the use of a metallic shield plate 15 interposed between the anode button 11 and the anode cap 16. However, these prior art attempts have been found unsatisfactory as an effective countermeasure for the X-radiation prevention for various reason, for example, by reason of difficulty in quality control, i.e., for 100% guarantee.
Furthermore, the fabrication of the anode button 11 by the use of a press work and the installation of the metallic shield plate 15 in the anode button require complicated and time-consuming procedures. This is due to the fact that the anode button 11 is relatively minute in size having a relatively small diameter, for example, about 10 mm. Specifically, the greater the thickness of the metallic shield plate 15, the lower the X-radiation emission. However, because the size of the anode button 11 is very small as hereinabove described, the available thickness of the metallic shield plate 15, as well as that of the metal plate for the anode button 11, are limited. Accordingly, it is a conventional practice that the anode button, effective to minimize the X-radiation emission as low as possible, is difficult to make.
Some other attempts are disclosed in numerous publications such as, for example, U.S. Pat. No. 3,969,647 issued Jul. 13, 1976; the Japanese Patent Publication (Examined) No. 62-26141 published Jun. 6, 1987; and the Japanese Laid-open Utility Model Publications No. 49-22847 published in 1974, No. 52-21763 published in 1977, No. 52-91753 published in 1977, No. 53-70863 published in 1978 and No. 54-55258 published in 1979. However, all of these prior art attempts require a complicated structure rendering the manufacture difficult and expensive.